Light-receiving circuit

ABSTRACT

A light-receiving circuit according to the present invention incorporates an amplifier 12 having differential output terminals, and it is intended that normal-phase and inverted-phase outputs (Vp, -Vp) of the amplifier 12 be supplied to a comparator 34 at an appropriate threshold level at all times. To achieve this intention, a peak-value voltage (Vi) of the normal-phase and inverted-phase outputs of the amplifier 12 is detected by a peak value detector 26, and is converted into a current by means of a voltage-current converter 28 having a mutual conductance of gm. The voltage-current converter 28 has differential output terminals, from which a normal-phase output (I +  =gm.Vi) and an inverted-phase output (I -  =gm.Vi) are produced, respectively. The currents (I + , I - ) are converted into voltages by means of resistors R10 and R12. The resultant currents are subtracted from the normal-phase and inverted-phase outputs (Vp, -Vp) of the amplifier 12, respectively, so that the normal-phase and inverted-phase outputs (Vp, -Vp) are shifted in level to the degree corresponding to the same voltage. The signals, thus shifted in level, are supplied to the normal-phase and inverted-phase input terminals of the comparator 34.

TECHNICAL FIELD

The present invention relates to a light-receiving circuit, and moreparticularly to a light-receiving circuit which converts a photo signalinto an electric signal and outputs the electric signal.

BACKGROUND ART

A conventional light-receiving circuit, which converts a photo signalinto an electric signal and outputs the electric signal, has such acircuit configuration as is shown in the "Light Receiver" disclosed inPublished Examined Japanese Patent Application (PEJPA) No. 63-25738.

More specifically, as is shown in FIG. 1, an amplifier 102 has its inputterminal 104 connected to a photodiode 100, and has its output terminal106 connected to the positive input terminal 110 of a comparator 108. Areference potential-generating circuit 112 is provided. The outputterminal 114 of this reference potential-generating circuit 112 isconnected to the negative input terminal 118 of a peak value-detectingcircuit 116 by way of resistor R100, and is further connected to theoutput terminal 106 of the amplifier 102 by way of resistors R102 andR104. The positive input terminal 120 of the peak value-detectingcircuit 116 is connected to node W, which is between resistors R102 andR104. The output terminal 122 of the peak value-detecting circuit 116 isconnected to node X by way of diode D100. Node X is connected to aconstant current source I100. Node X is further connected to thenegative input terminal 124 of the comparator 108 by way of node Y, towhich a capacitor C100 is connected. The output terminal 126 of thecomparator 108 is connected to the output terminal 128 of the receiver.

In the light-receiving circuit of the above circuit configuration, thethreshold value is automatically set to have an optimal level, withoutreference to a change in photo signal E supplied to the photodiode 100.

It is proposed that the amplifier 102 to which the photodiode 100 isconnected be replaced with an amplifier having differential outputterminals of normal and inverted phases. Such a proposal is made, forexample, in Japanese Patent Application No. 1-180717 entitled"Widely-Dynamic Light-Receiving Circuit" and Japanese Patent Application1-311334 entitled "Light-Receiving Circuit". When the amplifier havingdifferential output terminals is employed, it is possible to make thebest use of the amplitude of photo signal E, and the amplifier can havea wide band and be widely-dynamic.

The amplifier mentioned above has differential output terminals. Thus,even if the amplifier is incorporated in a circuit wherein the thresholdvalue is automatically set to have an optimal level, such as the circuitshown in FIG. 1, only one of its output terminals, a normal-phase one oran inverted-phase one, can be connected to a given circuit. Accordingly,the gain to be obtained is substantially 1/2 of the gain obtained in thecase where both output terminals are connected to that given circuit,and the amplifier fails to sufficiently achieve its advantages.

As may be understood from the above, the circuit mentioned above isdesigned on condition that the amplifier 102 to be incorporated has asingle-phase output terminal. The circuit is not designed for use withan amplifier having differential output terminals

As mentioned above, an amplifier having differential output terminals isnot suitable for incorporation into such a circuit configuration as isdisclosed in Published Examined Japanese Patent Application (PEJPA) No.63-25738 entitled "Light Receiver".

Accordingly, an object of the present invention is to provide alight-receiving circuit which has a circuit configuration permitting anamplifier with differential output terminals to be suitably incorporatedtherein, which employs a wide-band, widely-dynamic amplifier, and whichconstantly maintains the threshold value at an optimal level withoutreference to a change in the photo signal.

DISCLOSURE OF INVENTION

To achieve the above object, the light-receiving circuit of the presentinvention comprises:

an amplifier for amplifying a signal supplied from a light-receivingelement, the amplifier having normal-phase and inverted-phase outputterminals;

a peak value detector for detecting a peak value of an output of theamplifier, the peak value detector having normal-phase andinverted-phase input terminals which are connected to the normal-phaseoutput terminal of the amplifier, respectively;

a voltage-current converter having a normal-phase output terminal, aninverted-phase output terminal, and an input terminal which is connectedto an output terminal of the peak value detector;

a first resistor inserted between the normal-phase output terminal ofthe voltage-current converter and the normal-phase output terminal ofthe peak value detector;

a second resistor inserted between the inverted-phase output terminal ofthe voltage-current converter and the inverted-phase output terminal ofthe peak value detector; and

a comparator for comparing outputs of the amplifier with each other, thecomparator having normal-phase and inverted-phase input terminals whichare connected to the normal-phase and inverted-phase output terminals ofthe voltage-current converter, respectively.

In the light-receiving circuit having the above circuit configuration, apeak-value voltage of a normal-phase output of the amplifier and apeak-value voltage of an inverted-phase output of the amplifier aredetected by the peak-value detector. The peak-value voltages areconverted into currents by the voltage-current converter. By thisvoltage-current converter, a current corresponding to the normal-phaseoutput and a current corresponding to the inverted-phase are output. Thecurrent corresponding to the normal-phase output is converted into afirst voltage by the first resistor, while the current corresponding tothe inverted-phase output is converted into a second voltage by thesecond resistor. The first and second voltages are subtracted from thenormal-phase and inverted-phase outputs of the amplifier, respectively.By this subtraction, the normal-phase and inverted-phase outputs of theamplifier are shifted in level to the degree corresponding to the samevoltage. The two level-shifted signals are supplied to the normal-phaseand inverted-phase input terminals of the comparator, so that thesignals can be compared with each other with an optimal threshold levelat all times.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a circuit diagram showing a conventional light-receivingcircuit;

FIG. 2 is a circuit diagram showing a light-receiving circuit accordingto the first embodiment of the present invention;

FIG. 3 is a waveform chart showing the operation of the light-receivingcircuit of the first embodiment of the present invention;

FIG. 4 is a circuit diagram showing a light-receiving circuit accordingto the second embodiment of the present invention; and

FIGS. 5A through 5D are waveform charts each showing the operation ofthe light-receiving circuit of the second embodiment of the presentinvention.

BEST MODE OF CARRYING OUT THE INVENTION

Embodiments of the present invention will be described below, withreference to the accompanying drawings.

FIG. 2 is a circuit diagram showing a light-receiving circuit accordingto the first embodiment of the present invention, and FIG. 3 is awaveform chart showing the operation of the light-receiving circuit ofthe first embodiment.

As is shown in the Figures, the anode of a photodiode 10 is connected toa power supply Vcc, while the cathode thereof is connected to the inputterminal 14 of an amplifier 12. The normal-phase output terminal 16 ofthe amplifier 12 is connected to the normal-phase input terminal 20 of apeak value-detecting circuit 18 by way of node A, and the inverted-phaseoutput terminal 22 of the amplifier 12 is connected to theinverted-phase input terminal 24 of the peak value-detecting circuit 18by way of node B. The output terminal 26 of the peak value-detectingcircuit 18 is connected to the input terminal 30 of a voltage-currentconverter circuit 28. The normal-phase output terminal 32 of thevoltage-current converter circuit 28 is connected to the normal-phaseinput terminal 36 of a comparator 34 by way of node C. Theinverted-phase output terminal 38 of the voltage-current convertercircuit 28 is connected to the inverted-phase input terminal 40 of thecomparator 34 by way of node D. Nodes A and C are connected to eachother by means of resistor R10. Similarly, nodes B and D are connectedto each other by means of resistor R12. The output terminal 42 of thecomparator 34 is connected to the output terminal 44 of thelight-receiving circuit.

The operation of the light-receiving circuit having the above circuitconfiguration will be described, referring to calculation formulas.

Upon supply of a light signal E, the photodiode 10 outputs a receptionsignal I_(IN). In response to the supply of this reception signalI_(IN), the amplifier 12 outputs a normal-phase reception signal Vo+Vpand an inverted-phase reception signal Vo-Vp, both determined inaccordance with the reception signal I_(IN).

In the present specification, the reference symbol denoting each signalis associated with the voltage or current value of the signal. To bemore specific, in the reference symbols "Vo+Vp" and "Vo-Vp" respectivelydenoting the normal-phase and inverted-phase reception signals, "Vo"indicates a DC voltage component, and "Vp" indicates a signal voltagecomponent (signal amplitude).

Upon supply of the normal-phase reception signal Vo+Vp andinverted-phase reception signal Vo-Vp, the peak value-detecting circuit18 detects peak values of the voltages of the signals and outputs peakvalue signals Vi. The peaks of the voltages are, for example, twice ashigh as a signal amplitude voltage |Vp| (absolute value). Thus, thevoltage Vi of the peak value signals produced from the output terminal26 of the peak value-detecting circuit 18 is expressed as follows:

    Vi=2|Vp|                                 (1)

Upon supply of the peak value signals Vi, the voltage-current convertercircuit 28 outputs a normal-phase peak value signal I₊ and aninverted-phase peak value signal I₋, both determined in accordance withthe voltage Vi of the peak value signals.

Let it be assumed that the mutual conductance of the voltage-currentconverter circuit 28 is denoted by gm.

In this case, the normal-phase peak value signal current I₊ producedfrom the normal-phase output terminal 32 of the voltage-currentconverter circuit 28 is expressed as follows: ##EQU1##

On the other hand, the inverted-phase peak value signal current I₋produced from the inverted-phase output terminal 38 is expressed asfollows: ##EQU2##

The "Io" represents an initial current component which may flow throughthe current-voltage converter circuit 28 before the supply of the peakvalue signal Vi.

Assuming that each of resistors R10 and R12 has resistance R, thenormal-phase signal voltage V₊ supplied to the normal-phase inputterminal 36 of the comparator 34 is given by the following: ##EQU3##Similarly, the inverted-phase signal voltage V₋ supplied to theinverted-phase input terminal 40 is given by the following: ##EQU4##

In the case where the relationship between the mutual conductance gm andthe resistance R is represented by:

    R.gm=1/4,

formulas (4) and (5) can be respectively transformed as follows:

    V.sub.+ =Vo-R.Io-(1/2) |Vp|+Vp           (6)

    v.sub.- =Vo-R.Io+(1/2) |Vp|-Vp           (7)

In the right side of each of formulas (6) and (7), the first and secondterms represent a DC voltage component, and the first and second termsof the right side of formula (6) are equal to those of the right side offormula (7). The third term represents a DC voltage component whichvaries in accordance with the voltage Vp of the signal amplitude. Thefourth term represents the voltage of the signal amplitude itself.

As is shown in the waveform chart in FIG. 3, therefore, the voltageVo+Vp of the normal-phase reception signal output from the amplifier 12and the voltage Vo Vp of the inverted-phase reception signal also outputfrom the amplifier 12 are shifted in level by the voltage which is halfof that of the signal amplitude |Vp|, and are thus converted into anormal-phase signal of voltage V₊ and an inverted-phase signal ofvoltage V₋, respectively.

Due to the processing mentioned above, in the circuit of the firstembodiment, the comparator 34 can constantly compare the normal-phasesignal V₊ and the inverted-phase signal V₋ with each other at the levelcorresponding to the potential half that of the signal amplitude |Vp|,without reference to the value of the signal amplitude |Vp|.Accordingly, the comparator can compare the two signals with each otherwith an optimal threshold level at all times and produce an outputsignal V_(out).

The light-receiving circuit of the second embodiment will now bedescribed.

FIG. 4 is a circuit diagram showing the light-receiving circuit of thesecond embodiment of the present invention, and FIGS. 5A through 5D arewaveform charts each showing the operation of the light-receivingcircuit of the second embodiment. In these Figures, the same referencesymbols as those in FIGS. 2 and 3 are used to indicate the similar orcorresponding structural components, and a description will be givenonly of the different circuit configurations from those shown in FIGS. 1and 2.

As is shown in FIG. 4, in the circuit of the second embodiment, thenormal-phase output terminal 16 of an amplifier 12 is connected to nodeF, where the line branches out. Node F is connected to node G by way ofresistor R14. Similarly, the inverted-phase output terminal 22 of theamplifier 12 is connected to node H, where the line branches out. Node His connected to node G by way of resistor R16. The potential at node Gis set to be substantially 1/2 of the voltage applied between thenormal-phase and inverted-phase output terminals 16 and 22, for example,by providing resistors R14 and R16 with the same resistance. Theinverted-phase input terminal 31 of a voltage-current converter circuit28 is connected to node G by way of node J. The inverted-phase inputterminal 24 of a peak value-detecting circuit 18 is connected to node K.Nodes J and K are connected to each other by means of resistor R18. Acapacitor C10 is connected to node K.

The normal-phase input terminal 20 of the peak value-detecting circuit18 is connected to node A, which is connected to node F. The outputterminal 26 of the peak value-detecting circuit 18 is connected to theanode of a diode D10. The cathode of this diode D10 is connected to nodeL. The normal-phase input terminal 30 of the voltage-current convertercircuit 28 is connected to node L. Nodes L and K are connected to eachother. A constant current source I10 is connected to node L.

The operation of the light-receiving circuit having the above circuitconfiguration will be described, referring to calculation formulas.

Let it be assumed that the current value of the constant current sourceI10 is denoted by Ig.

Also, let it be assumed that the resistance of each of resistors R14 andR16 is denoted by R1, that the resistance of resistor R18 is denoted byRg, and that the relationship between resistance R1 and resistance Rg isgiven by the following:

    1/2(R1)>>Rg

Further, let it be assumed that the resistance of each of resistors R10and R12 is denoted by R, that the mutual conductance of thevoltage-current converter circuit 28 is denoted by gm, and that therelationship between resistance R and mutual conductance gm is given bythe following:

    R.gm=1/2

1. When light signal E is of a small value, i.e., when |Vp|≦Ig Rg, thepeak value-detecting circuit 18 does not operate. Thus, the inputvoltage Vi of the voltage-current converter circuit 28 satisfies thefollowing formula:

    Vi=Ig Rg (constant)                                        (8)

Output currents I₊ and I₋ of the voltage-current converter circuit 28are given by the following: ##EQU5##

Hence, input voltages V₊ and V₋ of the comparator 34 are given by thefollowing: ##EQU6##

In the right side of each of formulas (11) and (12), the first andsecond terms represent a DC voltage component. The third term alsorepresents a DC voltage component, but the DC voltage component of inputvoltage V₊ and that of input voltage V₋ are opposite in polarity. Due tothe opposite polarities, the DC voltage components serve as a guardvoltage when a small-value light signal is input or no light signal isinput.

2. When light signal E is of a large value, i.e., when |Vp|>Ig.Rg, thepeak value-detecting circuit 18 operates. Thus, the input voltage Vi ofthe voltage-current converter circuit 28 satisfies the followingformula:

    Vi=|Vp|                                  (13)

By similar calculation to that of the case where a small-value lightsignal is input, the input voltages V₊ and V₋ of the comparator 34 aregiven by the following:

    V.sub.+ =Vo-Io.R-(1/2) |Vp|+Vp           (14)

    V.sub.- =Vo-Io.R+(1/2) |Vp|-Vp           (15)

As may understood from formulas (14) and (15), as in the firstembodiment, normal-phase signal V₊ and inverted-phase signal V₋ can beconstantly compared with each other at the level corresponding to halfof potential |Vp|, i.e., at the level corresponding to (Vo-IR), withoutreference to the value of signal amplitude |Vp|. Therefore, thenormal-phase and inverted-phase outputs of the amplifier 12 can beconstantly compared in the comparator 34 with an optimal thresholdlevel, and output signal Vout can be produced as a result of thecomparison.

FIGS. 5A through 5D are waveform charts each showing the operation ofthe light-receiving circuit of the second embodiment.

FIG. 5(A) shows the waveform obtained when |Vp|=0, i.e., when no signalis input.

FIG. 5(B) shows the waveform obtained when the input signal satisfiesthe relation |Vp|<Ig.Rg.

FIG. 5(C) shows the waveform obtained when the input signal satisfiesthe relation |Vp|=Ig.Rg.

FIG. 5(D) shows the waveform obtained when the input signal satisfiesthe relation |Vp|>Ig.Rg.

Industrial Applicability

As has been described, the present invention can provide alight-receiving circuit which compares the outputs of an amplifier at anappropriate threshold level at all times, even if the amplifier hasnormal-phase and inverted-phase output terminals. Since, therefore, thelight-receiving circuit can employ a wide-band, widely-dynamicamplifier, the circuit is suitable for use in a light communicationsystem, for example.

I claim:
 1. A light-receiving circuit comprising:an amplifier having aninput terminal, a normal-phase output terminal, and an inverted-phaseoutput terminal, said amplifier amplifying an input signal which issupplied from a light-receiving element connected to the input terminal;a peak value detector having a normal-phase input terminal, aninverted-phase input terminal, and an output terminal, said normal-phaseinput terminal being connected to the normal-phase output terminal ofthe amplifier, said inverted-phase input terminal being connected to theinverted-phase output terminal of the amplifier, said peak valuedetector detecting a peak value of an output of the amplifier; avoltage-current converter having an input terminal, a normal-phaseoutput terminal, and an inverted-phase output terminal, said inputterminal being connected to the output terminal of the peak valuedetector; a first resistor inserted between the normal-phase outputterminal of the voltage-current converter and the normal-phase inputterminal of the peak value detector; a second resistor inserted betweenthe inverted-phase output terminal of the voltage-current converter andthe inverted-phase input terminal of the peak value detector; and acomparator having a normal-phase input terminal, an inverted-phase inputterminal, and an output terminal, said normal-phase input terminal beingconnected to the normal-phase output terminal of the voltage-currentconverter, said inverted-phase input terminal being connected to theinverted-phase output terminal of the voltage-current converter, saidcomparator comparing outputs of the amplifier with each other andproducing an output signal from the output terminal thereof.
 2. Alight-receiving circuit according to claim 1, wherein said peak valuecircuit detects a voltage determined by adding an amplitude voltagesupplied from the normal-phase output terminal of the amplifier to anamplitude voltage supplied from the inverted-phase output terminal ofthe amplifier, and supplies the voltage, thus determined, to thevoltage-current converter as a peak value.
 3. A light-receiving circuitaccording to claim 1, wherein said voltage-current converter has amutual conductance of gm, produces currents obtained in accordance withboth the mutual conductance gm and the voltage received as the peakvalue, and outputs the currents as a normal-phase output and aninverted-phase output, respectively.
 4. A light-receiving circuitaccording to claim 1, wherein said first and second resistors convertthe currents obtained by the voltage-current converter into voltages,respectively.
 5. A light-receiving circuit according to claim 1, whereinsaid voltage-current converter has a mutual conductance of gm, each ofsaid first and second resistors has a resistance of R, and the mutualconductance gm and the resistance R have a relationship given bygm×R=1/4.
 6. A light-receiving circuit comprising:an amplifier having aninput terminal, a normal-phase output terminal, and an inverted-phaseoutput terminal, said amplifier amplifying an input signal which issupplied from a light-receiving element connected to the input terminal;a first resistor and a second resistor which are inserted between thenormal-phase and inverted-phase output terminals of the amplifier; apeak value detector having a normal-phase input terminal, aninverted-phase input terminal, and an output terminal, said normal-phaseinput terminal being connected to the normal-phase output terminal ofthe amplifier, said inverted-phase input terminal being connected to apoint located between the first and second resistors, said peak valuedetector detecting a peak value of an output of the amplifier; avoltage-current converter having a normal-phase input terminal, aninverted-phase input terminal, a normal-phase output terminal, and aninverted-phase output terminal, said normal-phase input terminal beingconnected to the output terminal of the peak value detector, saidinverted-phase input terminal being connected to the point locatedbetween the first and second resistors; a third resistor insertedbetween the normal-phase output terminal of the voltage-currentconverter and the normal-phase output terminal of the peak valuedetector; a fourth resistor inserted between the inverted-phase outputterminal of the voltage-current converter and the inverted-phase outputterminal of the amplifier and a comparator having a normal-phase inputterminal, an inverted-phase input terminals, and an output terminal,said normal-phase input terminal being connected to the normal-phaseoutput terminal of the voltage-current converter, said inverted-phaseinput terminal being connected to the inverted-phase output terminal ofthe voltage-current converter, said comparator comparing outputs of theamplifier with each other and producing an output signal from the outputterminal thereof.
 7. A light-receiving circuit according to claim 6,wherein said peak value detector detects a voltage which is determinedby adding an amplitude voltage supplied from the normal-phase outputterminal of the amplifier to a voltage obtained by voltage division bythe first and second resistors, and supplies the voltage, thusdetermined, to the normal-phase input terminal of the voltage-currentconverter as a peak value.
 8. A light-receiving circuit according toclaim 6, wherein said first and second resistors have an equalresistance.
 9. A light-receiving circuit according to claim 6, furthercomprising a fifth resistor whose one end is connected to the pointlocated between the first and second resistors and whose another end isconnected to the inverted-phase input terminal of the peak valuedetector.
 10. A light-receiving circuit according to claim 6, whereinsaid voltage-current converter has a mutual conductance of gm, producescurrents obtained in accordance with both the mutual conductance gm anda voltage difference between the normal-phase and inverted-phase inputterminals thereof, and outputs the currents as a normal-phase output andan inverted-phase output, respectively.
 11. A light-receiving circuitaccording to claim 6, wherein said third and fourth resistors convertthe currents obtained by the voltage-current converter into voltages,respectively.
 12. A light-receiving circuit according to claim 6,wherein said voltage-current converter has a mutual conductance of gm,each of said third and fourth resistors has a resistance of R, and themutual conductance gm and the resistance R have a relationship given by:

    gm×R=1/2.


13. A light-receiving circuit according to claim 9, wherein each of saidfirst and second resistors has a resistance of R1, said fifth resistorhas a resistance of Rg, and resistance R1 and resistance Rg have arelationship given by:

    1/2×R1>>Rg.